Composite material with lubricating properties, process for their manufacture and antierosive components for a barrel weapon system which consists of these materials

ABSTRACT

The invention relates to a composite material with lubricating properties consisting of a solid polymeric matrix such as polystyrene or polymethyl methacrylate containing between 20 and 85% by weight of a liquid oil with lubricating properties, such as a silicone oil. 
     The material according to the invention is obtained by polymerization of an emulsion of the said oil in a monomer such as styrene or methyl methacrylate monomer in the presence of a surface-active agent which is a two- or three-block block copolymer in which one repeat unit is compatible with the said monomer. 
     The materials according to the invention are used to form antierosive components for ammunition intended for barrel weapon systems and in particular wads (8) for shells (1).

The present invention relates to the field of composite materials withlubricating properties, employed as antierosive components in ammunitionfor barrel weapon systems.

More precisely, the invention relates to a new type of compositematerial with lubricating properties, a process for its manufacture andits application to the production of antierosive components intended forequipping ammunition for barrel weapon systems.

The problem of combating the erosion of the barrels of weapon systems isa problem which assumes increasing importance with the development ofweapon systems with a high rate of fire.

Erosion of the barrels occurs for two reasons: a mechanical reason dueto the friction of the projectile against the inner surface of thebarrel and a chemical reason due to the chemical corrosion of the innersurface of the barrel, caused by the hot gases generated by thecombustion of the propellant charge.

The mechanical erosion can be remedied quite well by fitting theprojectile with an external girdle of a soft and ductile material suchas copper, which limits the abrasion of the inner surface of the barrel.This solution is described, for example, in French Patent No. 2,036,458.

On the other hand, the fight against the corrosion caused by the hotgases turns out to be more tricky.

As a general rule, in order to reduce this corrosion, attempts are madeto insert into the ammunition, between the propellant charge and thebase of the projectile, a body with lubricating properties which will beentrained by the hot gases to be deposited onto the inner surface of thebarrel so as to form a protective screen both from a chemical standpointand from a thermal standpoint.

A first type of body with lubricating properties and employed for thispurpose consists of mixtures of waxes and of inorganic fillers such astalc, titanium oxides or tungsten. A solution of this type which isdescribed, for example, in European Patent Application No. 0,235,533,has a major disadvantage: when the ammunition is stored in hotconditions, the waxes tend to melt and this then leads to the ammunitionfunctioning badly when fired.

A second type of body with lubricating properties and used for thispurpose consists of sheets or pouches of synthetic material which arefilled with lubricating liquid. Thus, U.S. Pat. No. 4,334,477 describesan ammunition in which the propellant charge is surrounded by a sheet ofpolyacrylonitrile which has absorbed a water-glycol mixture. U.S. Pat.No. 4,282,813, in its turn, describes an ammunition in which a siliconeoil encapsulated in a polyester film has been placed between the chargeand the base of the projectile. While a solution of this second type nolonger has the disadvantage of being heat-sensitive, it has thedisadvantage, on the other hand, of not offering a controlled anduniform release of the lubricating liquid in all directions at the timeof firing.

The problem of the fight against chemical corrosion by the hot gases inthe barrel weapon systems has therefore not been solved in a fullysatisfactory manner at the present time.

The object of the present invention is precisely to propose a solutionwhich answers this problem in a fully satisfactory manner.

The subject of the invention relates, therefore, to a composite materialwith lubricating properties, characterized in that it consists of asolid polymeric matrix containing, in a homogeneous manner, throughoutits mass, a dispersion of liquid oil droplets with lubricatingproperties.

Preferably the solid polymeric matrix consists essentially ofpolystyrene or of polymethyl methacrylate and the liquid oil consistsessentially of a silicone compound.

The invention also relates to a process for obtaining a materialaccording to the invention by preparing, in a first step, an emulsion ofthe liquid oil with lubricating properties in a liquid phase of a liquidmonomer capable of being polymerized by radical polymerization, themonomer not being a solvent for the said liquid oil, in the presence ofa surface-active agent and by polymerization, in a second step, of theemulsion by addition of a chemical initiator of radical polymerizationor by use of a radiation of electromagnetic type, characterized in thatthe surface-active agent is a block copolymer comprising two two-blockor three-block repeat units, in which one repeat unit is compatible withthe liquid oil and in which the other repeat unit is compatible with themonomer, the surface-active agent having a number-average molecular massof between 500 and 500,000.

Lastly, the invention relates to antierosive components for barrelweapon systems, and especially to wads for artillery shells,characterized in that they consist of a material according to theinvention.

The advantage of the material according to the invention lies in thefact that, when subjected to a compression, it functions really like a"wet sponge", releasing the oil with which it is loaded in a controlledand uniform manner in all the directions of ejection. Thus, anantierosive component made of a material according to the invention,inserted between the propellant charge and the base of the projectile ofan ammunition for barrel weapon systems, will be compressed under theeffect of the pressure rise at the time of the firing of the propellantcharge and will eject the liquid oil with lubricating properties in auniform manner over the whole inner surface of the barrel of the weapon.The compacted solid matrix itself is destroyed by the hot gases.

The invention is described in detail below with reference to FIGS. 1 to4.

FIG. 1 shows, seen in section, an ammunition comprising an antierosivecomponent according to the invention in a first geometricalconfiguration.

FIG. 2 shows, seen in section, an ammunition comprising an antierosivecomponent according to the invention in a second geometricalconfiguration.

FIG. 3 shows, diagrammatically, an experimental device intended tomeasure the efficiency of the ejection of oil by the material accordingto the invention.

FIG. 4 shows, diagrammatically, an experimental device intended tosimulate the operation of an antierosive component according to theinvention in a barrel weapon system.

The invention relates, therefore, to a composite material consisting ofa solid polymeric matrix containing, in a homogeneous manner, throughoutits mass, a dispersion of droplets of a liquid oil with lubricatingproperties. The polymer forming the solid matrix must not be soluble inthe liquid oil. Since this polymer is obtained by radical polymerizationof an emulsion of the said oil in a liquid monomer, the monomer,precursor of the polymer, must not itself be soluble in the liquid oileither. Various solid polymers may therefore be suitable, so long asthey meet the two conditions just stipulated. Polymers of vinyl oracrylic type are suitable within the scope of the present invention.Polystyrene and polymethyl methacrylate will be mentioned as preferredpolymers.

The liquid oil employed may be any liquid oil with lubricatingproperties, natural or synthetic, which remains in the liquid state inthe conditions of storage of the ammunition and in the conditions of usein a weapon with a high rate of fire. The oil must therefore remainliquid between -40° C. and +250° C. at least, and preferably between-65° C. and +300° C. An oil consisting essentially of a siliconecompound will be advantageously employed. The preferred siliconecompounds will be those corresponding to the general formula: ##STR1##in which: R¹ and R², which are identical or different, denote the methylgroup or the phenyl group, and

n is an integer of between 10 and 10,000 and preferably between 50 and1,000.

A composite material according to the invention may contain up to 85% byweight of liquid oil, dispersed and occluded within the solid matrix. Onthis topic, it should be noted that, while the material according to theinvention operates as a "wet sponge" when it is subjected tocompression, at rest this material has the appearance and theconsistency of a true solid and that it can be wrought or machined likea solid as long as the droplets of oil are sufficiently fine.

The invention also relates to a process for the manufacture of thematerial according to the invention. In a first step, this processconsists of preparing an emulsion of the said liquid oil withlubricating properties in a continuous liquid phase of a liquid monomercapable of being polymerized by radical polymerization. As has alreadybeen stated above, the said monomer must not be a solvent for the liquidoil and, when polymerized, must not produce a polymer soluble in thelatter. Monomers of vinyl, styrenic, pyridinic, acrylic or methacrylicstructure will be advantageously employed. Styrene and methylmethacrylate will be particularly preferred as monomers.

The said polymerizable liquid monomer may be employed by itself or mixedwith a polymer of the same nature as the final solid matrix in order toimprove emulsion stability. Up to 30% by weight of polymer may thus beadded, relative to the weight of monomer employed.

The emulsion is produced by stirring by means of any known apparatus inthe presence of a surface-active agent. In a characteristic manner, thesurface-active agent is a block copolymer containing two-block repeatunits A-B or three-block repeat units A-B-A or B-A-B, in which onerepeat unit is compatible with the liquid oil and in which the otherrepeat unit is compatible with the said liquid monomer. "Compatibility"between two constituents, within the meaning of the present invention,means the fact that these two constituents are miscible in the form of asingle homogeneous phase. The surface-active agent may have anumber-average molecular mass of between 500 and 500,000 and preferablybetween 1,000 and 200,000.

The surface-active agent will be advantageously chosen from two-block orthree-block block copolymers of the following type:polyoxyethylene-polydimethylsiloxane,polyoxypropylene-polydimethylsiloxane, polymethylmethacrylate-polydimethylsiloxane, polystyrenepolydimethylsiloxane andpolydimethylsiloxane-polyoxyethylene-polyoxypropylene.

Block copolymers of this kind are described, for example, in EuropeanPatent Application No. 0,038,979 and in French Patent Applications Nos.2,314,932 and 2,204,640.

According to a preferred embodiment of the invention, a styrene-basedmatrix will be chosen, coupled with block copolymers of thepolystyrene-polydimethylsiloxane type, or an acrylic matrix coupled withblock copolymers of the polyoxyethylene-polydimethylsiloxane,polyoxypropylene-polydimethylsiloxane andpolydimethylsiloxane-polyoxyethylene-polyoxypropylene type.

Although, according to the invention, block copolymers are preferred asa surface-active agent, it will also be possible to employ graftcopolymers as surface-active agents within the scope of the presentinvention.

The emulsion may contain between 1 and 20% by weight of surface-activeagent relative to the weight of the liquid monomer, but preferably thisquantity will be between 1 and 10% by weight.

When the emulsion of oil in the polymerizable monomer is produced, thepolymerization of the emulsion is initiated in a second step by theaddition of a chemical initiator of radical polymerization or by the useof a radiation of electromagnetic type capable of initiating thepolymerization, such as, for example, a UV radiation or an electronbombardment. The use of a radiation of electromagnetic type is limited,however, to products of small thickness, chiefly products in the form ofsheets or plates. An azo compound, a percarbonate or a peroxide such asbenzoyl peroxide will be advantageously employed as initiator of radicalpolymerization. The quantity of polymerization initiator will be between0.1% and 5% by weight relative to the weight of the said polymerizablemonomer, and preferably in the region of 1%.

Lastly, in certain cases, it is possible to add to the emulsion acrosslinking agent such as divinylbenzene or carbon tetrabromide, so asto create bridging in the polymeric matrices employed.

The polymerization of the emulsion is generally carried out at atemperature of between 20° C. and 150° C., depending on the means ofinitiation employed. The polymerization time may be between 2 and 120hours in the case of polymerization employing a chemical initiator, orof the order of a minute in the case of polymerization employing aradiation of electromagnetic type.

Since the emulsion may contain up to 85% by weight, relative to itsentire weight, of liquid oil, the process thus makes it possible toobtain materials with a solid polymeric matrix containing between 20%and 85% of liquid oil relative to their total weight.

The finer the emulsion, the more finely distributed the liquid oil willbe in the solid polymeric matrix, a condition which will be obtainedproportionally better the higher the concentration of surface-activeagent in the emulsion and the lower the molecular mass of thesurface-active agent.

Lastly, the invention relates to the application of the materials justdescribed to the production of antierosive components for ammunitionintended for barrel weapon systems. These elements may have geometricconfigurations which are usually adopted in the case of antierosivecomponents already known.

FIG. 1 depicts a sectional view of an ammunition equipped with acomponent of this kind. This figure shows a shell body 1 with a base 2crimped to one of its ends. The base 2 carries a primer 3. The end ofthe shell body 1 away from the base 2 carries a constriction 4 in whicha projectile 5 is crimped. The shell body 1 is charged with a charge 6of propellant powder. In its part close to the projectile 5, the innersurface of the shell body 1 is surrounded by an antierosive component 7,in the form of a sleeve, consisting of a composite material withlubricating properties according to the invention.

A preferred embodiment of the invention is depicted in FIG. 2, viewed insection. This embodiment relates to an ammunition similar to that shownin FIG. 1, in which the antierosive component is in the form of a wad 8inserted between the propellent charge 6 and the base of the projectile5.

As already stated above, at the instant of firing, the antierosivecomponent 7 or 8 will be compressed and expelled following theprojectile 5 by the hot gases generated by the combustion of the charge6 and will eject the oil which it contains in all directions against theinner surface of the barrel of the weapon, which will contribute tolowering the temperature of the latter by virtue of the endothermicdecomposition of the oil and will thus limit its corrosion of a chemicalnature.

The examples which follow illustrate certain possibilities of use of theinvention without limiting its scope.

The tests have been carried out with two different silicone oils:

Oil A: polymethylphenylsiloxane: oil with a viscosity of 710 centipoises(cP), that is 0.710 Pa s

Oil B: polydimethylsiloxane: oil with a viscosity of 1,000 cP, that is 1Pa s.

EXAMPLE 1

In this example, an attempt has been made to manufacture a compositematerial with lubricating properties without employing a surface-activeagent.

12 g of styrene monomer and 0.12 g of benzoyl peroxide (that is 1% byweight relative to the monomer) are added to 18 g of oil A.

The mixture is homogenized by stirring with a centrifugal turbinestirrer rotating at 4,500 revolutions per minute for 5 minutes.

Polymerization is then ensured without stirring in an oven controlled at80° C. for 24 hours.

A compact material, with an oily feel, is thus obtained, exhibitingspherical cells varying between 20 and 50 microns (micrometres) indimensions and containing 58% by weight of silicone oil.

EXAMPLE 2

The experiment of Example 1 is restarted, but this time in the presenceof a surface-active agent.

11.5 g of styrene monomer, 0.12 g of benzoyl peroxide and 0.30 g of apolystyrene-polydimethylsiloxane block copolymer are added to 18 g ofoil A. This two-block copolymer is characterized by a number-averagemolecular mass of 56,000 in the case of its polystyrene block and of50,000 in the case of its polydimethylsiloxane block.

The emulsion is produced by stirring for 5 minutes with a centrifugalturbine stirrer rotating at 4,500 revolutions per minute.

Polymerization of the system is carried out in the same conditions asthose of Example 1.

A compact material is thus obtained, more rigid and drier than thematerial of Example 1. The use of the surfactant results in a greateruniformity in the size of the cells, which contain 60% of silicone oiland whose dimensions are between 5 and 25 microns (micrometers).

EXAMPLE 3

26.7 g of styrene monomer, 0.3 g of benzoyl peroxide, 1.5 g ofdivinylbenzene and 1.5 g of a polystyrene-polydimethylsiloxane blockcopolymer, as surface-active agent, are added to 70 g of oil A. Thistwo-block copolymer is characterized by a number-average molecular massof 14,000 in the case of the polystyrene block and of 37,500 in the caseof the polydimethylsiloxane block.

The emulsion is produced by stirring for 5 minutes with a centrifugalturbine stirrer rotating at 5,000 revolutions per minute.

The polymerization is carried out at 80° C. for 24 hours withoutstirring.

A compact material is thus obtained, containing 70% by weight of oilpresent in the cells, whose size is between 10 and 30 microns(micrometers).

The addition of divinylbenzene makes it possible to obtain a polymerlattice, which doubles the rupture strength of the material at 20° C.relative to that of the material obtained in Example 2.

EXAMPLE 4

26.7 g of styrene monomer, 0.3 g of benzoyl peroxide, 1.5 g ofdivinylbenzene and 1.5 g of polystyrene-polydimethylsiloxane blockcopolymer, as surface-active agent, are added to 70 g of oil B. Thistwo-block copolymer is defined by a number-average molecular mass of5,250 in the case of the polystyrene block and of 10,000 in the case ofthe polydimethylsiloxane block.

The conditions under which the emulsion is produced and thepolymerization carried out are similar to those of Example 3.

A compact material containing 70% by weight of oil B in a polystyrenematrix crosslinked with divinylbenzene is thus obtained. The size of thecells containing the oil is between 10 and 35 microns (micrometers).

EXAMPLE 5

28.2 g of methyl methacrylate, 0.3 g of benzoyl peroxide, 1.5 g ofpolydimethylsiloxane-polyoxyalkylene two-block block copolymer assurface-active agent, are added to 50 g of oil B. The polyoxyalkyleneblock itself is made up of a polyoxyethylene block followed by apolyoxypropylene block.

This copolymer is defined by a number-average molecular mass of 7,500,in a proportion of 21% on a numerical basis in the case of thepolydimethylsiloxane block, 21% on a numerical basis in the case of thepolyoxyethylene block and 58% on a numerical basis in the case of thepolyoxypropylene block.

The emulsion is produced by stirring for 6 minutes with a centrifugalturbine stirrer rotating at 5,000 revolutions per minute. This emulsionis then polymerized at 60° C. for 48 hours without stirring.

A very hard and homogeneous block is thus obtained, containing 62% byweight of silicone oil in spherical cells, whose size is between 15 and45 microns (micrometers).

EXAMPLE 6

The material obtained in Example 3 and containing 70% by weight of oil Awas tested in a dynamic compression device shown diagrammatically inFIG. 3.

An anvil 12, in the bottom of which a sample 13 of the material to betested is placed, is arranged in a manometric enclosure 11. A plunger 14movable inside a positioning ring 15 can bear on the sample 13.

A quantity of propellant powder 16 is placed inside the manometricenclosure. This quantity of powder is that needed to obtain a pressureof 100 MPa inside the enclosure 11.

After testing, the solid residue originating from the sample 13 nowweighs only 35% of its initial weight. The missing 65% represent theweight of oil A released during the dynamic compression.

EXAMPLE 7

The material obtained in Example 4, which contains 70% by weight of oilB was tested in the device shown diagrammatically in FIG. 4.

The device employed is intended to simulate a shot fired in a barrelweapon system. It consists of a manometric enclosure 21 extended by achamber 22 which opens into a barrel 23 communicating with adecompression enclosure 24 closed, at the beginning of the test, by abursting disc 25 calibrated at 280 MPa. A thermocouple 26 arranged 0.2mm from the inner surface of the barrel 23 makes it possible to obtainthe temperature of the inner surface of the barrel 23. A ring 27 made ofthe material to be tested is arranged in the chamber 22 so as to permita direct communication between the barrel 23 and the enclosure 21. Aquantity of propellant powder 28 is arranged inside the enclosure 21.This quantity is that needed to obtain a pressure slightly above 280 MPain the enclosure 21 and the barrel 23.

A test with the powder by itself without a ring 27 and a test with thepowder and a ring 27 are carried out.

The combustion of the powder 28 in the enclosure 21 results in apressure rise up to 280 MPa, at which pressure the bursting disc yields,causing hot gases to flow over the ring 27. The change in thetemperature indicated by the thermocouple is recorded as a function oftime and the heat impulse transmitted (quantity of heat supplied perunit of surface area) is calculated.

The results have been as follows:

    ______________________________________                                                      Maximum                                                                       temperature                                                                            Heat impulse                                                         (°C.)                                                                           (joule/cm.sup.2)                                       ______________________________________                                        Powder by itself                                                                              25.6       4.37                                               Powder with ring 27                                                                           22.6       3.17                                               ______________________________________                                    

A 27% decrease in the heat impulse transmitted is observed in the caseof a firing in the presence of an antierosive ring according to theinvention, the associated loss in weight of the ring being 43.6%.Furthermore, measurements carried out at various points of the barrel 23show a decrease in the heat impulse transmitted at all these points,which demonstrates that the spraying of oil over the inner surface ofthe tube does take place in a uniform manner in all the directions ofejection.

We claim:
 1. A process for the manufacture of a composite material withlubricating properties consisting of a solid polymeric matrix containingthroughout its mass, a uniform dispersion of liquid silicone oildroplets with lubricating properties, which consists essentially ofpreparing, in a first step, an emulsion of said liquid silicone oil withlubricating properties in a liquid phase of a monomer wherein saidmonomer is styrene or methylmethacrylate and said matrix containspolystyrene or polymethyl methacrylate, said liquid silicone oil beinginsoluble in said monomer, in the presence of a surface-active agent andin a second step polymerizing said emulsion wherein said surface-activeagent is a block copolymer containing two-block or three-block repeatunits, one repeat unit being compatible with said liquid oil and theother repeat unit being compatible with said monomer, saidsurface-active agent having a number-average molecular mass of between500 and 500,000 and isolating said composite material of particle sizebetween 5 and 45 microns.
 2. The process according to claim 1, whereinsaid emulsion contains 20-85% of oil by weight of said emulsion.
 3. Theprocess according to claim 2, wherein said emulsion contains between 1and 20% by weight of said surface-active agent relative to the weight ofsaid liquid monomer.
 4. The process according to claim 3, wherein saidsurface-active agent is a two-block or three-block block copolymer whichis a member selected from the group consisting ofpolyoxyethylene-polydimethylsiloxane,polyoxypropylene-polydimethylsiloxane, polymethylmethacrylate-polydimethylsiloxane andpolydimethylsiloxane-polyoxyethylene-polyoxypropylene.
 5. The processaccording to claim 1, wherein said silicone compound has the formula##STR2## in which: R¹ and R² are identical or different, and are methylor phenyl, andn is an integer between 10 and 10,000.
 6. The processaccording to claim 5, wherein said silicone compound ispolymethylphenylsiloxane of viscosity 710 centipoises (cP), orpolydimethylsiloxane of viscosity 1,000 cP.
 7. The process according toclaim 1, wherein a cross-linking agent is added to said emulsion in theamount up to 30% by weight with respect to the weight of said monomer.8. The process according to claim 7, wherein said cross-linking agent isdivinyl benzene.
 9. The process according to claim 1 wherein thepolymerization in said second step is a radical polymerization promotedby an initiator.
 10. The process according to claim 1 wherein thepolymerization in said second step is promoted by radiation of theelectromagnetic type.